i'm with onsmart. since jan or feb, if i remember correctly. no problems here, i rather like them. i don't recall any slowdowns or downtime (but then again, i'm not on my website a lot myself). don't really know what the support is like since i've had no need to contact them. i found mixed reviews when simply searching "onsmart" on the web. some say they're great, others say they suck.
i guess it's really up to you. i can attest to them, i've had no problems since i started there.